Densification of porous bodies

ABSTRACT

A porous preform body is infiltrated with carbon by a thermal gradient process using a multi-portion heating element to heat the body. Selected portions of the heating element are supplied with power to create the thermal gradient between different areas of the preform body.

The invention relates to densification of porous bodies. One particularinstance is in the densification of porous structural components such asbrake discs or pads for aircraft, which typically are made of a porouscarbon body infiltrated by carbon.

Many different methods are known for the deposition of a carbon orceramic matrix on a fibrous substrate of carbon or ceramic fibres. In athermal gradient process the densification is started at the inside ofthe substrate or preform and progresses outwards until the whole articleis densified. Because the exterior pores of the preform do not becomeblocked until the centre of the preform is densified, it is found thatdensification can proceed at a higher rate, reducing processing timesand cost compared to the isothermal method of densification.

In all processes where a fibrous substrate is being densified thecontrol of deposition conditions, such as temperature and pressure, isimportant to ensure the deposition reaction continues at the requiredrate in order to produce a deposit of the required structure. Where amatrix of carbon is being deposited to manufacture friction discs foraircraft brakes, it can be desirable that the deposit is a graphitisablecarbon in order to heat treat the composite to maximise the thermalproperties. (The graphitisable form of carbon is often referred to asrough laminar and can be distinguished from the non-graphitisable smoothlaminar form by microscopic examination using polarised light).

In thermal gradient deposition heating is commonly by induction heatingof a graphite susceptor see, e.g. U.S. Pat. No. 5,348,774 whichdiscloses a process in which the fibrous substrate is in contact with agraphite mandrel which is heated by the electromagnetic field from aninduction coil. The fibrous substrate is then heated by conduction fromthe graphite mandrel. The main problem with this method of establishingthe thermal gradient in the preform is the requirement for a susceptorcore which might only be usable once if damage is caused during removalfrom the densified composite.

Another method of producing a thermal gradient in a fibrous substratefor densification is by chemical vapour infiltration (CVI) and isdescribed by J. J. Gebhardt et al in a paper entitled “Formation ofcarbon-carbon composite materials by pyrolitic infiltration”, PetroleumDerived Carbons ACS Ser. No. 21 6/73, pages 212 to 217. In the method ofGebhardt the thermal gradient is maintained by a high rate of gas flowcooling the outer surface of the substrate and by heating only a smallvolume of the cylindrical substrate in the induction coil, densificationof the whole being achieved by moving the substrate inside the inductioncoil. The substrate is manufactured from graphite fibres which havesufficient electrical conductivity to enable heating of the substrate bydirect coupling with the induction coil. Practical constraints on theprocess make it difficult to apply on an industrial scale and it isunsuitable for other substrate geometry, such as a disc.

U.S. Pat. No. 5,390,152 discloses a method of densification in oneversion of which (FIG. 6), a porous body is placed on a support betweenso-called horizontal coils or pancake coils. It is suggested that thespacing between the turns of the coil may be non-uniform in someinstances. It is also suggested that as the preform porous body beginsto densify the frequency of the applied current may have to be increasedto compensate for a decrease in resistivity or the current may need tobe increased for decreased resistance.

It is one object of the invention to provide an improved method ofdensification in which the heating is controlled to improve the rate ofdeposition after graphitisation.

In one aspect there is provided a method of infiltrating a porouspreform body with carbon by heating the body having different portionsto a temperature at which carbon-containing gas is crackable to depositcarbon in the pores thereof, the method comprising placing the bodyadjacent a multi-portion heating element so that the portions thereofalign with corresponding portions of the body, and supplying power tothe heating element to heat the body whilst exposing the body to acrackable carbon-containing gas; characterised by selectively energisingdifferent portions of the multi-portion heating element to create athermal gradient between one heated body portion and the or eachadjacent body portion so as to selectively deposit carbon in the poresof each different heated body portion to provide a graphitisable roughlaminar structure.

Preferably adjacent portions of the multi-portion heating element areused to heat adjacent portions of the body to different temperatureswhereby to create a thermal gradient between the respective portions ofthe body.

DE-A-4142261 discloses a method of infiltrating a porous body with a gasstream by means of a heating element the shape of which can be matchedto that of the body. The document teaches that for a body having anirregular shape the heating element may be made of plates having acorresponding shape and that such plates may have independent powersupplies. There is no suggestion of the creation of a thermal gradientfor infiltration.

Preferably the heating element is generally circular. In one variationthe heating element comprises a base comprising an induction heatingtube shaped into substantially concentric coils, and means are presentto power each coil individually. Means may be present to power somecoils in combination. In another variation the heating element is madeup of vertically aligned coils, arranged to heat layers within thepreform.

The porous body will typically be a fibrous preform, of any type. Thefibres will be carbon or other suitable fibre.

The infiltrating gas will be one which may be infiltrated by a chemicalvapour infiltration technique. The gas may conveniently be provided by agas vapour or a liquid precursor.

In another aspect the invention provides a method of method ofinfiltrating a porous preform body with carbon by heating the bodyhaving different portions to a temperature at which carbon-containinggas is crackable to deposit carbon in the pores thereof, the methodcomprising placing the body adjacent a multi-portion heating element sothat the portions thereof align with corresponding portions of the body,and supplying power to the heating element to heat the body whilstexposing the body to a crackable carbon-containing gas; characterised byselectively energising different portions of the multi-portion heatingelement to create a thermal gradient between one heated body portion andthe or each adjacent body portion so as to selectively deposit carbon inthe pores of each different heated body portion.

Preferably the infiltration of the selected portion is continued until apredetermined level has been achieved following which the process isrepeated to adjacent selected portions in succession whereby toinfiltrate substantially all of the body to substantially uniform levelin a relatively short time period.

In order that the invention may be well understood it will now bedescribed by way of illustration only with reference to the accompanyingdiagrammatic drawings in which:

FIG. 1 is a diagrammatic representation of one densification apparatusof the invention; and

FIGS. 2 to 4 illustrate different methods of using the heating elementin apparatus of FIG. 1.

The densification apparatus comprises a carbon deposition chamber 1housing a heating base 2 connected by connectors 3 to a source ofelectric current for induction heating. Carbon-containing gas is pumpedvia an inlet 4 and is passed over the base 2 to an outlet 5. The basicstructure of the chamber is known and therefore will not be described indetail. In use, a fibrous preform P is placed on the base 2 or on anintermediate support, heated and a cyclohexane which is a liquidprecursor is pumped through. Of course other liquids and gases may beused.

The element 2 fundamentally comprises a set of metal coils, typicallycopper, arranged substantially concentrically. The coils may be circularor of other suitable shape. The coils may have a round or rectangularcross-sectional shape. (The coils may be evenly or non-uniformly spacedapart). Each coil is independently powered by induction, e.g. by beingcoupled to a source of electric current, not shown. The coils may havean individual source operating at a selected frequency, or one generatormay be present, together with a switching system to heat up one or morecoils. In use, each coil is separately powered to heat up just theoverlying portion of the preform, so creating a thermal gradient. Oncethat portion has been infiltrated the process is repeated on anotherportion of the preform.

In the embodiment of FIG. 2, the element is a base 2 made up of threeconcentric coils 101,102,103. A porous fibrous preform P having acentral hole is placed on the base, optionally on a plate or support,not shown. Induction heating is applied to each coil which in turn heatsup the overlying zone of the preform shown as A,B,C respectively. Forexample zone A is heated by coil 101, zone B by coil 102 and zone C bycoil 103. By heating up individual coils, the heating of the preform isconfined to the zone immediately above. The heating will establish onehot zone adjacent an unheated one, so creating a thermal gradient. Byheating coils in succession the operator can move the front fordensification across the preform, typically from the centre to theperiphery. The operator can also heat the coils in differentcombinations so as to concentrate the densification in selected zones bysupplying power to each coil (or combination of coils) to heat up therespective zone(s) to the temperature at which carbon-containing gas canbe cracked the operator has better control of the densification. Ourinvestigations have established that this control is important in thedeposition of a carbon matrix for the formation of the graphitisablerough laminar structure required for good thermal properties aftergraphitisation.

In the embodiment of FIG. 3, the base has five coils 201,202,203,204 and205, each associated with a zone A,B,C,D,E. Because of the mutliplicityof coils different coils may be energised together, e.g. first coil 203,followed by coils 202 and 204 and then by coils 201 and 205. In this waythe preform is progressively heated outwards from the centre.

In the embodiment of FIG. 4 the coils 301,302 and 303 are arrangedvertically to heat horizontal zones or layers A,B,C within the preform.Again the coils may be energised selectively. In this case the preform Pis mounted on a supporting plate of suitable electrical and heatinsulation properties.

Our evaluations have established that using the method of the inventionone can infiltrate at a preform at a relatively faster rate to provide abody of controlled microstructure. The method allows the operator tochange the temperatures easily, so that he has better control over thethermal gradient.

The invention is not limited to the embodiments shown. The distancebetween coils need not be uniform. The preform need not have a hole. Thecoils may be of any suitable cross-sectional shape. A preform may beplaced on a support. The preform may be heated from above as well asbelow. A number of deposition chambers may be arranged with identicalbases, heated by a common generator or generators. The deposit may becarbon or a ceramic matrix. Layers of heating elements and preforms maybe stacked in any sequence for selective heating.

What is claimed is:
 1. A method of infiltrating different portions of aporous preform body with carbon by heating the body to a temperature atwhich carbon-containing gas is crackable to deposit carbon in the poresthereof, the method comprising placing the body adjacent a multi-portionheating element shaped into substantially concentric coils so that thecoils thereof align with corresponding portions of the body, andsupplying power to the heating element to heat the body whilst exposingthe body to a crackable carbon-containing gas whereby carbon isdeposited in the pores of the heated portion; and including the step ofselectively energising different coils of the multi-portion heatingelement to create a thermal gradient between one heated body portion andthe or each adjacent body portion so as to propagate an infiltrationfront from the centre to the periphery of the preform body andselectively deposit carbon in the pores of each different heated bodyportion to provide a graphitisable rough laminar structure.
 2. A methodaccording to claim 1 comprising providing, as the heating element, aninduction heating element made up of horizontally spaced coils,disposing the coils about the body and heating vertically definedportions thereof.
 3. A method according to claim 1 further comprisingproviding each portion of the heating element with a respective powersupply.
 4. A method according to claim 1 further comprising supplyingpower to the portions of the heating element from a common power source,power being supplied to a selected portion of the heating element byvirtue of switch means.
 5. A method according to claim 1, comprisingcontinuing the heating and infiltration of one selected portion of thebody until a predetermined level of carbon deposition has been achievedfollowed by performing the method with adjacent selected portions insuccession whereby substantially all of the body has carbon depositedtherein to a substantially uniform level.